Japanese male and female patients with T1DM and age 18 to 75 years, with inadequate glycemic control on insulin defined as HbA1c ≥ 7.5% and ≤ 10.5% at screening visit. As a condition of enrollment, subjects must be on a total daily insulin dose of ≥ 0.3 U/kg/day for at least 3 months prior to the screening visit. The study design of Part B is a randomized, open-label, 2 arm, parallel-group design. 140 Japanese subjects in total will be randomized in a 1:1 ratio into one of the two treatment arms; dapagliflozin 5 mg or dapagliflozin 10 mg.